1. Field of the Invention
This invention relates to a method and apparatus for the atomization of fluids in rapidly spinning cup.
2. Description of the Previously Published Art
Powdered metals of fine sizes are required for many applications, and for some of these applications it is desirable that the mean size of the particles be less than 20 micrometers, sometimes even substantially less than this value. There are many processes for making metal powders, but those of greatest interest are those which atomize (i.e. fragment) a molten metal. Atomization processes are preferred over other methods because the powder particles can then have compositions identical to that of the melt from which they were formed, whereas many other types of processes are more restricted in the range of compositions they can produce, and they are generally more costly than atomization.
There are many different atomization processes, of which the most widely practiced are gas atomization and water atomization. Gas atomization can be done in many different ways, but all of them consist of fragmenting a stream of molten metal by causing it to interact with jets of gas flowing from one or more nozzles. Water atomization consists of fragmenting a stream of molten metal by causing it to interact with water ejected at high speed from one or more nozzles. Metal powders are also produced by oil atomization, which is identical in principle to water atomization, but which, under similar operating conditions, produces coarser powder than water atomization. The reason for this is that oils are more viscous than water and thus they exit from the nozzles at lower speeds than does water. Thus the oil streams are less energetic than water and hence less able to fragment the molten metal.
U.S. Pat. No. 4,394,332 to Raman et al at Battelle discloses one method to accelerate oil (and other fluids) to high speed so as to make it an effective atomizing medium. This method consists of containing a limited volume of atomizing fluid within a rimmed cup and spinning the cup at high speed about its central axis. This method is called the Rapidly Spinning Cup or RSC process. Molten metal is dripped or injected into the rapidly spinning fluid and is atomized by it. This method can produce fine powders and the powders can have desirable properties such as low levels of contamination, but the method suffers from the disadvantage of producing only limited amounts of powder.
This disadvantage is because the Raman et al process is a batch process. The amount of powder which can be produced is limited by the volume of fluid which can be contained by the cup. The volume of powder generated must be substantially less than the volume of atomizing fluid so as to prevent overheating of the fluid, so even with large spinning cups the weight of powder produced could not reasonably exceed a few tens or hundreds of pounds.
U.S. Pat. No. 4,405,535 to Raman et al at Battelle discloses two further variations of the rapidly spinning cup embodiment. In FIG. 1 the RSC is essentially the same as in U.S. Pat. No. 4,394,332 discussed above. The change is in the manner by which the molten metal is added. In the ""332 patent the apparatus is a crucibleless device because the metal is in the form of a solid bar. The end inside the spinning cup is heated so the metal at that tip end melts and drips into the RSC. In the ""535 patent a vessel with a surrounding heating means serves as a crucible which provides the molten metal supply. This device in FIG. 1 is only suitable for operation as a batch process.
The embodiment in FIG. 2 is an attempt to devise a continuous process. Prior art processes were not continuous because they used a fixed volume of oil contained within the cup. The powder also accumulated within the cup and displaced part of the oil, so that the volume of oil actually decreased with time. The amount of melt was restricted to a volume substantially less than that of the oil, because of the heat transferred from the melt to the oil.
U.S. Pat. No. 4,405,535 in FIG. 2 seeks to overcome this by permitting oil to exit from the side of the cup. A hole is made in the side of the wall of the spinning cup to permit the quench fluid and formed particulates to flow from the cup when the process is in operation. The quench liquid is cooled and recirculated back into the RSC. The size of the hole is kept small enough that in relation to the speed of rotation of the cup and the pump capacity of the pump an adequate supply of quench fluid is maintained in the cup. This, however, is an ineffective approach because it does not provide an efficient means for the constantly added oil to flush powder from the cup. With a single hole on the cup perimeter as shown in the patent, or even with a multiple of holes, the powder will accumulate on the inner vertical wall of the cup and be held there by centrifugal force.
A later article by Erich et al xe2x80x9cBattelle Plan Commercialisation of Two-Stage Spinning Cup Atomisation Processxe2x80x9d Metal Powder Report, Vol. 42, No. Oct. 10, 1987 pg. 698-700, discloses two types of spinning cup atomizations. The single stage RSC is illustrated in FIG. 1 herein and consists of a rotating cup with a liquid quenchant. Under the action of centrifugal forces, the liquid forms a layer on the walls of the cup during rotation. A stream of molten metal to be atomized is directed onto the rotating quenchant where it is sheared and pulverized into small droplets. The two stage process has the primary liquid metal stream disintegrated into fine ligaments or large liquid metal droplets by low pressure gas or conventional centrifugal atomization. Thus the molten metal is broken up into droplets before contacting the atomizing fluid in the spinning cup. However, here again the process is only a batch process.
Additional research on the RSC process has been conducted at the Naval Research Laboratory. Ayers et al in xe2x80x9cCounter Rotating Fluid Atomization of Tinxe2x80x9d 2nd International Conference on Rapidly Solidified Materials, March 7-0 1988, disclose an atomization process which combines the rapidly spinning cup process with a centrifugal atomization. The atomizer is positioned within a rapidly spinning cup that rotates in the opposite direction. The fine streams of molten metal which issue from the inner cup strike the fluid contained within the outer cup and they are atomized and quenched by it. The outer cup has a rim which prevents the melt from spinning over the top. Again this is a batch process.
Cooper et al in xe2x80x9cCounter Rotating Fluid Atomizationxe2x80x9d International Journal of Powder Metallurgy, Vol. 28, No. 3, 1993, pg 215-226, disclose studies made on atomization of molten streams of tin injected into rapidly moving oil contained with the rim of a spinning cup. The method consisted of forcing the melt by centrifugal force through fine orifices on the perimeter of a rotating cup centered within the oil-containing cup. The two cups were counter rotating. The powder is recovered from the quench fluid by first lowering the speed of the outer cup to about 200 rpm and raising the cover of the vacuum vessel, thereby removing the inner cup. Most of the oil is then carefully pipetted off, leaving the remaining oil and the powder held against the wall of the spinning cup. This powder/oil slurry is then recovered by pipetting with the thin oil layer being replenished repeatedly with kerosene to assure full recovery, of the powder. After complete removal of the oils using appropriate solvents, the powder is subjected to size analysis. Clearly this is a batch process.
Cooper at al in xe2x80x9cEvaluation of Atomization by the Rapidly Spinning Cup Processxe2x80x9d International Journal of Powder-Metallurgy, Vol. 30, No. 1, 1994, pg 77-89, disclose a parametric study of the atomization of molten tin injected into oil contained within the rim of a rapidly spinning cup. The liquid metal was introduced as a stream by ejecting it from a small orifice in a tube using pressurized gas. Because the oil was contained within the rim the process was a batch process.
3. Objects of the Invention
It is an object of this invention to produce fine powders in tonnage quantities.
It is a further object of this invention to provide a Rapidly Spinning Cup (RSC) process which operates in a continuous mode.
It is a further object of this invention to provide a RSC process where the cup can be rotated at higher speeds.
It is a further object of this invention to provide a RSC process where the atomizing liquid is not present in the cup during the beginning of the production run when the cup is being accelerated and during the end of the production run when the cup is being decelerated.
It is a further object of this invention to provide a RSC apparatus which does not need to be designed to accommodate the harmonic vibrations which occur at different frequencies in a spinning vessel which contains a liquid.
It is a further object of this invention to provide a RSC process where the cup can be spun at higher speeds so that it can produce finer size powders.
It is a further object of this invention to provide a RSC process where the cup is spinning at such a high speed that it requires only a thin film of atomizing fluid on the inside of the cup.
These and further objects of the invention will become apparent as the description of the invention proceeds.
This invention makes possible the production on a semi-continuous basis (that is, in multi-ton lots) of fine powders from molten metals and alloys. The process employs a shallow cup rotating at high speeds. Once the high speeds are obtained an atomizing fluid such as water, oil or any other hydrocarbon is fed to the cup to form a thin sheet or layer which is distributed on the inner surface of the cup and which is accelerated to speeds essentially the same as that of the spinning cup. Within the cup a stream or spray of molten metal is propelled into this thin sheet of atomizing fluid. The metal interacts with the fluid and is fragmented or broken down into many small droplets which are quenched by the atomizing fluid and solidified into fine powder. These powders can be continuously removed and recovered. Because the atomizer can be spun at higher speeds, it can produce finer powders.
A further preferred embodiment is to add a preatomizer between the stream of molten metal and the spinning cup. A mechanical impact preatomizer, for example, has rotating impeller blades which break up the molten stream into a series of droplets that will be directed to the atomizing liquid film on the inner wall of the spinning cup. Other embodiments include the use of a gas atomizer or a centrifugal atomizer.